Optocouplers in computer switching power supplies

An optocoupler is generally composed of three parts: light transmission, light reception, and signal amplification. The input electrical signal drives a light-emitting diode (LED) to emit a certain wavelength of light, which is received by the photodetector to generate a photocurrent, which is further amplified and output.

This completes the conversion of electricity-optical-electricity, thus playing the role of input, output and isolation. Because the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectionality, so it has good electrical insulation ability and anti-interference ability. In addition, because the input terminal of the optocoupler is a low-impedance component that operates in the current type, it has a strong common-mode rejection ability.

Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation element in long-term transmission of information. It is used as an interface device for signal isolation in computer digital communication and real-time limitations.

The optocoupler function of the computer switching power supply is mainly electrical isolation and feedback sampling voltage, so as to achieve the duty cycle of the control switch tube, achieve the purpose of precision voltage stabilization, and make the output of the switching power supply stable and smooth DC. When the optical coupling is damaged, the switching power supply will not work properly.

The function of the optocoupler is mainly used to isolate transmission and control, and can prevent interference and fault amplification. If the optocoupler is damaged, there will definitely be a control or transmission abnormality (mainly depending on the effect of the optocoupler), resulting in the power supply not working or the protection function fails.

To put it simply, the role of negative feedback.

The role of optocouplers in switching input and output circuits is to transmit switching signals and to achieve electrical isolation between circuits and to filter out noise and interference.

1) The input impedance of the photocoupler is very small, only a few hundred ohms, while the impedance of the interference source is larger, usually 105 106. According to the principle of voltage division, even if the amplitude of the interference voltage is large, the noise voltage fed to the input of the photocoupler will be very small, and only a very weak current can be formed, which cannot make the diode emit light because there is not enough energy, so it is suppressed.

2) There is no electrical connection between the input loop and the output loop of the photocoupler, and there is no common ground; The distribution capacitance is very small, and the insulation resistance is very large, so it is difficult for all kinds of interference noise on one side of the loop to be fed to the other side through the photocoupler, so as to avoid the generation of interference signals of common impedance coupling.

3) The photocoupler can play a good role in safety, even when the external equipment fails, or even when the input signal line is shorted, it will not damage the instrumentAppearance. This is because the input and output circuits of optocoupler devices can withstand high voltages of several thousand volts.

4) The response speed of the photocoupler is extremely fast, and its response delay time is only about 10 s, which is suitable for occasions with high requirements for response speed.

The working principle and examples of optocoupler circuits.

The optocoupler is composed of a light-emitting diode and a phototransistor or photodiode. Add power to the input end, and the output end will be switched on, which generally plays the role of isolation between the control circuit and the power circuit, such as using 12V or lower voltage to control the 220V power supply.

In some laboratories or high-demand occasions, for the safety of experimenters, the input power supply of the experiment is generally isolated from the mains power by a 1:1 power frequency transformer, so that the laboratory experimenters will not be at risk of electric shock no matter which line they touch, because the isolated power supply is not connected to the earth. In industrial control equipment, it is sometimes required to isolate the power ground between the two systems, such as isolating the ground noise, isolating the high common-mode voltage, etc., and a DC converter with a transformer is used to separate the two power supplies from each other so that they are independent of each other.

Opto-decoupled isolated feedback is a simple, low-cost way to use a general isolated power supply. However, there has been no in-depth study on the various connection modes and their differences for optocoupler feedback. Moreover, in many cases, due to the lack of deep understanding of the working principle of optocoupler, the optocoupler method is chaotic, which often leads to the failure of the circuit to work normally.

In this study, the working principle of optocoupler will be analyzed in detail, and several typical connection methods of optocoupler feedback will be compared.

1 Several common connection methods and how they work.

The photocoupler has small size, long service life, wide operating temperature range, and strong anti-interference performance. The absence of contacts and the complete electrical isolation of input and output make it widely used in a wide range of electronic devices. Optocouplers can be used in circuits such as isolation circuits, load interfaces, and various household appliances.

Optocoupler models commonly used for feedback include TLP521, PC817, etc. The TLP521 is used as an example to describe the characteristics of this type of optocoupler.

The primary side of TLP521 is equivalent to a light-emitting diode, the larger the primary current if, the stronger the light intensity, and the larger the current IC of the secondary triode. The ratio of the secondary triode current IC to the primary diode current if is called the current amplification coefficient of the optocoupler, which changes with temperature and is greatly affected by temperature.

TL431 is usually chosen in combination with TLP521 for feedback. In this case, the operating principle of TL431 is equivalent to a voltage error amplifier with an internal reference of V, so between its pins 1 and 3, a compensation network should be connected.

The general structure is a light-emitting diode and a phototransistor, and the light-emitting diode will make the transistor turn on the phototransistor to achieve signal isolation.

The two pins of the diode are the anode and cathode of the light-emitting diode, and the other two pins are the collector and emitter of the photosensitive triode.

The usefulness of the TV is not well understood, but it is also used as a signal coupling.

The working principle and examples of optocoupler circuits.

The working principle and examples of optocoupler circuits.